1. Field of the Invention
The present invention relates to polyglycerols. It also relates to processes of preparing polyglycerols in which glycerol and/or a polyglycerol such as diglycerol is reacted with glycidol in the presence of an activated carbon catalyst.
2. Description of the Related Art
Polyglycerols are widely used as moisturizing agents, thickening agents, and plasticizers. They are also used as raw materials for polyglycerol fatty acid esters that are used as emulsifiers for food additives, cosmetics, and medical materials.
Materials for food additives must comply with various specifications or standards, such as the specifications specified by the Joint FAO/WHO Expert Committee on Food Additives established by the Food and Agriculture Organization (FAO) and the World Health Organization (WHO). According to the specifications, for example, glycerol fatty acid esters for food additives must have a total content of diglycerol, triglycerol, and tetraglycerol of 75% or more, and a total content of polyglycerols each having carbon atoms equal to or more than heptaglycerol of 10% or less. In addition, they are preferably free from compounds harmful to the human body.
Polyglycerols have been conventionally prepared by heating and condensing glycerol in the presence of an alkali catalyst. This preparation process, however, invites by-production of branched-chain and cyclic glycerols, in addition to straight-chain glycerols, and the composition of the product cannot be significantly controlled. In addition, such cyclic glycerols are harmful to the human body and should be minimized.
Possible alternates for this preparation process are a process of allowing α-monochlorohydrin of glycerol or diglycerol with an alkali metal glycerolate and/or alkali metal diglycerolate (for example, U.S. Pat. No. 4,973,763) and a process of reacting glycerol-α-monochlorohydrin with epichlorohydrin in the presence of an acid or a compound with an acid reaction (for example, U.S. Pat. No. 4,992,594). These processes, however, use chlorine-containing compounds such as monochlorohydrin and epichlorohydrin as starting materials, and chlorine atoms cannot be fully removed from products polyglycerols. Thus, they are not desirable in view of safety, health, and environment.
To avoid these problems, proposed is a process of preparing polyglycerols using chlorine-free materials, in which glycerol or a polyglycerol is reacted with glycidol in the presence of a sulfonic acid ion-exchange resin catalyst (for example, Japanese Unexamined Patent Application Publication (JP-A) No. Sho 61-140534). The product polyglycerol mixture prepared according to this process, however, has a high degree of polymerization, and no polyglycerol product having a composition complying with the specifications has been obtained. In addition, the process requires repeated ion exchange procedures, thus inevitably has complicated steps, and invites high cost as an industrial preparation process.
As is described above, no polyglycerol composition has yet been developed which has a composition complying with the specifications, is free from chlorine and other harmful substances, and is suitable as a food additive.